Throbin-activated human platelets cause agglutination of trypsinized, formalinized bovine erythrocytes. This lectin activity of stimulated platelets was blocked by galactosamine, glucosamine, mannosamine, lysine, and arginine, but not by N-acetylated sugars, other neutral sugars, or other amino acids. Inhibitors of the thrombin-induced lectin activity also blocked thrombin-induced platelet aggregation. It appears that a membrane surface component that has lectin activity mediates platelet aggregation.